Recently, active research works have been made on organic EL devices. As a basic configuration, the organic EL device includes a hole injecting electrode, a thin film formed thereon by evaporating a hole transporting material such as triphenyldiamine (TPD), a light emitting layer deposited thereon of a fluorescent material such as an aluminum quinolinol complex (Alq.sub.3), and a metal electrode or electron injecting electrode formed thereon from a metal having a low work function such as magnesium. Such organic EL devices are attractive in that they can achieve a very high luminance ranging from several 100 to several 10,000 cd/m.sup.2 with a drive voltage of approximately 10 volts.
Organic EL devices are very sensitive to moisture. Contact with moisture can cause serious problems including separation between the light emitting layer and the electrode layer, deterioration of constituent materials, formation of non-light-emitting zones known as dark spots, and reduction of the luminous area. As a result, the EL devices fail to maintain light emission of the desired quality.
One known solution to this problem is to shield an organic EL multilayer structure on a substrate from the exterior by tightly securing a gas-tight casing or sealing layer to the substrate so as to enclose the EL structure, as disclosed in JP-A 5-36475, 5-89959, and 7-169567.
Despite the provision of such casings or sealing layers, moisture gradually penetrates in the interior with the lapse of drive time whereby the devices deteriorate in several aspects, such as a drop of light emission luminance, generation or enlargement of dark spots, and reduction of the luminous area. Eventually the devices become unusable because of failure of light emission.
It was also proposed to accommodate the organic EL structure in a gas-tight casing in which a desiccant is contained. For example, JP-A 3-261091 discloses diphosphorus pentoxide (P.sub.2 O.sub.5) as the desiccant. However, P.sub.2 O.sub.5 tends to absorb moisture and becomes liquid (deliquescence) and thus forms phosphoric acid, which can be detrimental to the organic EL structure. The introduction of P.sub.2 O.sub.5 into the casing requires a careful limited procedure. This technique is impractical.
JP-A 6-176867 discloses fine powdery solid desiccants such as zeolite, active alumina, silica gel, and calcium oxide. The desiccants of the type that physically adsorbs water, typically zeolite, release the once adsorbed water by the heat associated with the light emission of the organic EL device. The lifetime of the device is not fully long.
JP-A 9-148066 discloses desiccants in the form of compounds capable of chemically adsorbing moisture and maintaining a solid state even after moisture absorption, for example, alkali metal oxides, alkaline earth metal oxides, sulfates, and metal halides. Because of the chemical adsorption, once water is adsorbed, these compounds no longer release the water. The lifetime of the device is increased, but not to a satisfactory extent.